Abstract: Formation of silicon oxynitride was observed during silicon nitride whisker synthesis from silica, carbon and nitrogen. The silicon oxynitride formation was limited to a bottom area of the charged powders and found both in whisker and powder. The possible reason for this localized Si 2 N 2 O formation is analysed, based on the effect of gas phase composition on the phase stability among β-Si 3 N 4 , β-SiC and Si 2 N 2 O. The phase stability is closely related to the ratio in the gas phase. To suppress Si 2 N 2 O formation, the ratio must be lower than that of the phase boundary in the Si 2 N 4 /Si 2 N 2 O equilibrium, whereas it must be higher than that of the phase boundary in the SiC/Si 2 N 2 O equilibrium. The formation of silicon oxynitride during silicon nitride whisker formation was caused most likely by fluctuation in the ratio in the gas phase surrounding the lower area.